chore: Use --no-cache-dir flag to pip in Dockerfiles, to save space

Using "--no-cache-dir" flag in pip install ,make sure dowloaded packages
by pip don't cached on system . This is a best practise which make sure
to fetch ftom repo instead of using local cached one . Further , in case
of Docker Containers , by restricing caching , we can reduce image size.
In term of stats , it depends upon the number of python packages
multiplied by their respective size . e.g for heavy packages with a lot
of dependencies it reduce a lot by don't caching pip packages.

Further , more detail information can be found at

https://medium.com/sciforce/strategies-of-docker-images-optimization-2ca9cc5719b6

Signed-off-by: Pratik Raj <rajpratik71@gmail.com>

proxyhub/Dockerfile

@@ -24,7 +24,7 @@ ADD http://download.dnscrypt.org/dnscrypt-proxy/dnscrypt-proxy-1.4.3.tar.gz /tmp
 WORKDIR /tmp/pkgs/
 RUN chmod +x /usr/local/bin/jq
 RUN python get-pip.py
-RUN pip install supervisor shadowsocks
+RUN pip install --no-cache-dir supervisor shadowsocks
 RUN tar xzf libsodium-1.0.2.tar.gz && \
     cd libsodium-1.0.2 && \
     ./configure && \